This invention relates generally to optical waveguides in polymer materials and, more particularly, to optical waveguides formed employing fabrication techniques compatible with high density interconnect (HDI) fabrication techniques, including the use of adaptive lithography to compensate for component misposition.
As disclosed in commonly assigned Eichelberger et al. U.S. Pat. No. 4,783,695, and related patents such as those referenced hereinbelow, a high density interconnect structure offers many advantages in the compact assembly of electronic systems. For example, a microcomputer which incorporates between thirty and fifty chips, or even more, can be fully assembled and interconnected on a single substrate which is two inches long by two inches wide by 50 mils thick. This structure is referred to herein as an "HDI structure", and the various previously-disclosed methods for fabricating HDI structures are referred to herein as "HDI fabrication techniques".
Very briefly, in systems employing this high density interconnect structure, a ceramic substrate is provided, and individual cavities, or one large cavity having appropriate depths at the intended locations of the various chips, are prepared. Various components are placed in their desired locations within the appropriate cavity and adhered to the substrate by means of a thermoplastic adhesive layer.
A multi-layer high density interconnect (HDI) overcoat structure is then built up to electrically interconnect the components into an actual functioning system. To begin the HDI overcoat structure, a polyimide dielectric film, such as Kapton polyimide available from E. I. du Pont de Nemours & Company, about 0.0005 to 0.003 inch (12.5 to 75 microns) thick is pretreated to promote adhesion and coated on one side with Ultem.RTM. polyetherimide resin available from General Electric Company, or another thermoplastic, and laminated across the tops of the chips, other components and the substrate, with the Ultem resin serving as a thermoplastic adhesive to hold the Kapton film in place. (Kapton is a trademark of E. I. duPont de Nemours & Co., and Ultem is a trademark of General Electric Company.) Exemplary lamination techniques are disclosed in commonly assigned Eichelberger et al. U.S. Pat. No. 4,933,042.
The actual as-placed locations of the various components and contact pads thereon are typically determined by employing optical imaging techniques. Via holes are adaptively laser drilled in the Kapton film and Ultem adhesive layers in alignment with the contact pads on the electronic components in their actual as-placed positions. Exemplary laser drilling techniques are disclosed in commonly assigned Eichelberger et al. U.S. Pat. Nos. 4,714,516 and 4,894,115, and Loughran et al. U.S. Pat. No. 4,764,485.
A metallization layer is deposited over the Kapton film layer and extends into the via holes to make electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned form individual conductors during its deposition, or may be deposited as a continuous layer and then patterned using photoresist and etching techniques. The photoresist is preferably exposed using a laser which, under program control, is scanned relative to the substrate to provide an accurately aligned conductor pattern upon completion of the process. Exemplary techniques for patterning the metallization layer are disclosed in commonly assigned Wojnarowski et al. U.S. Pat. Nos. 4,780,177 and 4,842,677, and Eichelberger et al. U.S. Pat. No. 4,835,704 which concerns an "Adaptive Lithography System to Provide High Density Interconnect". Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system as disclosed in U.S. Pat. No. 4,835,704.
As an alternative to electrical interconnections, various forms of waveguide structures in optically transparent dielectrics such as polyimides have been proposed. Examples are disclosed in the literature and include R. Selvaraj, H. T. Lin and J. F. McDonald, "Integrated Optical Waveguides in Polyimide for Wafer Scale Integration," Journal of Lightwave Technology, Vol. 6 No. 6 June 1988, pages 1034-1044; and B. L. Booth, "Low Loss ChannelWaveguides in Polymers," Journal of Lightwave Technology, Vol. 7, No. 10, October 1989, pages 1445-1453. Related techniques are disclosed in Booth et al. U.S. Pat. No. 4,883,743.